degrouping stl file objects on blender
Use the Separate function. 

Enter Edit Mode (Tab)

Highlight (select) the object you are breaking apart, press space, type "separate" (or use the keyboard shortcut P) then pick "Loose Parts" from the list of options.







To help you select the parts, you can select one vertex of or the specific item you want to separtate and press CtrlL to select all of the connected vertices. Then separate using P

A different way to select different parts is using Box select (CtrlB